Have telegraphed very high powered attacks even in random battles.
Sure 90 % of the time this Imp just casts fire, but sometimes he starts to charge 'Demon Gate' instead.
You don't kill the imp very fast and auto spam some other enemy to death?
There's his big brother whos just disabled 'Escape'.
That sniper in the back is using 'Laser sight' instead of 'Quick shot' and you just auto attacked the beefy guy in front? Boom. One hit KO next turn unless you counterattack.
Mage casts damage reversal causing physical damage to bounce back tenfold? Woops, you killed yourself.
You get afflicted with 'Doom'? Better be at full health next turn or you die.
Or Mimic enemies. Oh, it's a goblin. Nope, it's the Mimic that looks like a goblin it but is just a tidbit different in his moveset ...
Requiring enemies to be defeated in a specific order: As long as the blood golem lives, the artificer is immortal, but the blood golem is immune to physical damage, but the Artifcer keeps casting reflect if not attacked.
Having linchpin enemies that end the encounter if defeated, but have way more HP. (Debuff, and piledrive that SO

.
Enemies that alternate between 'vulnerable and offensive' and 'defensive and countering'.
Friendly 'enemies' that need to survive. These don't even need to be living things. "Why did you attack the pillar holding up the roof?"
They want to spam attack? They lose.